Generator Backup Fuel Calculator

Frequently Asked Questions

How do I estimate generator fuel use?

Fuel per hour ≈ load fraction × full-load burn rate. A 7 kW gasoline generator burns ≈ 0.75 gal/h at full load and ≈ 0.45 gal/h at half load; 8 hours at half load uses ≈ 3.6 gal.

How do gasoline, propane, and diesel compare?

Rule-of-thumb burns per kW of output: gasoline ≈ 0.1 gal/kWh, propane ≈ 0.13 gal/kWh (less energy per gallon), diesel ≈ 0.07 gal/kWh. Diesel is most efficient and propane stores longest.

What does it cost per kWh to run a generator?

Cost/kWh = fuel $/gal ÷ kWh per gallon. Gasoline at $3.50 producing 10 kWh/gal = $0.35/kWh - roughly 2–3× grid power and a key reason hybrid solar + small generator setups dominate off-grid.

How long can I run on a tank?

Runtime hours = usable tank gallons ÷ gal/h at expected load. A 7-gal tank at 0.5 gal/h gives 14 h; leave 10–15% margin for cold-start, surge, and accuracy of the fuel gauge.

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This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator Works

A backup or off-grid generator is rated at a continuous output (kW), but its fuel burn depends on how loaded it is - a 7 kW unit at 25% load uses much less than at full load, and not in a straight line. Most piston gensets are tuned for peak efficiency near 70–80% load; running them at <25% load is inefficient (and bad for them long-term - "wet stacking" in diesels). This tool takes the generator's rated kW, your average load percentage, total runtime hours, and fuel price, and estimates fuel burn rate, total fuel consumed, total cost, and the resulting cost per kWh delivered - the number to compare against solar + battery alternatives.

Formula / Method

Empirical full-load burn rates (per kW of rated output): gasoline ≈ 0.18 gal/h/kW, propane ≈ 0.20 gal/h/kW (propane has lower energy density than gasoline so the gallon-per-hour rate is higher), diesel ≈ 0.10 gal/h/kW. Part-load adjustment: rate ≈ full-load rate × (0.25 + 0.75 × load %) - this captures the "no-load" baseline burn of roughly 25% of full-load and the near-linear rise with load. Total gallons = rate (gal/h) × hours. Energy delivered = rated kW × load % × hours. Cost per kWh = total cost ÷ energy. For liters, multiply gallons by 3.78541. These are planning approximations - consult the manufacturer's spec sheet for the actual fuel-consumption curve.

Worked Example

A 7 kW gasoline portable genset at 50% load (3.5 kW) for 24 hours during an outage. Full-load burn = 0.18 × 7 = 1.26 gal/h. Part-load adjustment: 1.26 × (0.25 + 0.75 × 0.5) = 1.26 × 0.625 ≈ 0.79 gal/h. Over 24 hours: 18.9 gallons. At $3.85/gal: $72.78 in fuel. Energy delivered = 3.5 × 24 = 84 kWh. Cost per kWh = $72.78 ÷ 84 = $0.866/kWh - roughly 3× the typical residential retail rate of $0.30, and 5–10× the levelised cost of solar + battery. That math is why long-duration generator runs are economic punishment, and why the first morning after a 3-day outage everyone is at the gas station.

Common Mistakes

Using the full-load fuel rate when running at 25–50% load - overestimates burn by 30–50% (or underestimates if you used a no-load estimate instead).

Forgetting oil - gasoline generators want an oil change every 50–100 hours; that adds $5–$15 per outage event.

Sizing on the "starting watts" surge instead of running watts - chronically loaded at 90%+ the genset wears out fast and runs hot.

Ignoring fuel storage limits - gasoline degrades in 3–6 months without stabiliser; propane is shelf-stable for years; diesel can grow algae if stored damp.

Running the generator unloaded for hours - diesel especially needs >30% load to avoid wet stacking and exhaust gunk.

Tips & FAQ

Gas, propane, or diesel? Propane stores indefinitely and a 500-gallon tank gives meaningful autonomy; gasoline is everywhere but degrades; diesel is most efficient and longest-lived but cold-start cranky and more expensive up front. For preparedness, propane wins. What size for whole-house? 7–10 kW covers essentials (fridge, well, furnace blower, lights, some 120 V loads). 14–20 kW for AC and a range. 22–26 kW for everything. Inverter vs. conventional: inverter generators throttle their engine to load and burn 40–60% less fuel at part-load - the right call for long outages with variable load. Solar / battery hybrid: a small battery bank with the generator as backup-of-backup is dramatically cheaper per kWh than running the generator full time - the genset becomes the kWh source of last resort, charging the bank in short, high-load bursts when efficient. Sound: a portable open-frame at 50 ft is 65–75 dB; an enclosed standby is 55–65 dB; an inverter genset can be under 60 dB - meaningful for neighbour relations during week-long outages. Tier 4 / EPA: small portable diesels are largely out of the US market post-2014 due to Tier 4 emissions; that is why propane and gasoline dominate residential. Maintenance: exercise standby gensets weekly; carb gum is the #1 portable-genset failure mode. These figures are planning estimates - for safety-critical sizing always consult the manufacturer's certified fuel-consumption curve.

Generator Backup Fuel Calculator

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